1. Field of the Invention
The present invention relates to a cooling water passage structure for an engine wherein a cooling water passage is formed between a pair of exhaust ports.
2. Description of the Related Art
A cylinder head of an engine has an exhaust port for passing exhaust gas therethrough, and thus the portion of the cylinder head surrounding the exhaust port is heated to high temperatures, as is commonly known. Especially, in a four-valve engine having a pair of exhaust ports formed in a cylinder head thereof, knocking is liable to occur as the heat of the exhaust gas accumulates in the region between the exhaust ports, which is a primary cause of lowering in the engine performance. To eliminate the inconvenience, a cooling water passage structure having a cooling water passage formed between two exhaust ports has been proposed, for example, in Examined Japanese Patent Publication No. H02-43025 (hereinafter referred to as the patent document).
In the cooling water passage structure disclosed in the patent document, a pair of cooling water passages are formed by drilling so as to cross each other in the form of the letter X and located between the ignition plug and the pair of exhaust ports. The cooling water passages permit the heat of the exhaust gas passing through the exhaust ports to escape to the cooling water in the cooling water passages, thereby preventing heat transfer to the ignition plug. To form the cooling water passages by mechanical machining, namely, by drilling, however, a special machining step is required, giving rise to a problem that additional labor and time accompanying the machining step leads to an increase in the manufacturing cost.
Such cooling water passages located between the exhaust ports may be formed by casting as cast holes, but there is no sufficient space between the two exhaust ports. Especially in the case of a small-sized engine, it is difficult to form a cooling water passage between two exhaust ports by casting.